Driveline and Universal Joints
Driveshafts and U-joints
A single-Cardan U-joint produces a small cyclic speed change, so each shaft has two joints whose operating angles must be equal and opposite (within roughly 1/2 degree) so the fluctuations cancel. The yokes on a shaft must be in phase - ears at both ends parallel - unless the OEM specifies otherwise for a two-piece shaft with a CV or center bearing. Mark all shaft-to-yoke relationships before disassembly.
The operating angle must be small enough to avoid vibration but never zero: a joint running dead straight does not rotate its needle bearings, causing brinelling and flat spotting. Sagging springs, wrong spring rate, overload, or a changed ride height alter the angles and create vibration.
The center (carrier) bearing needs a sound rubber insulator and correct shims. Slip splines must be lubricated and the vent clear; a blocked slip joint forces thrust into the transmission output bearing.
Diagnostic traps:
- Vibration that tracks engine rpm in neutral is engine, clutch, or flywheel; vibration that tracks vehicle speed is driveline, axle, tire, or wheel.
- A once-per-revolution vibration at wheel speed is tire or wheel; higher-order or second-order vibration suggests driveline.
- A clunk on acceleration and deceleration indicates worn U-joints, a loose slip spline, or excessive differential backlash.
- A seized U-joint shows rust, blue heat marks, or blocked grease passages; replace it, do not grease it.
Rule of thumb: check shaft runout and U-joint looseness in all four planes and confirm ride height is per spec before blaming the shaft for a vibration complaint.